Hitherto, there has been serious problem in air pollution and harmful influence on the Earth environment derived from exhaust gas of internal-combustion engines, such as an automobile. In order to lighten the problem, many methods have been proposed, for example an improvement of a structure or combustion mode of internal-combustion engines, and employment of a filter, a removal of an impurity in fuel and the like. However, some of them have made a sacrifice of the performance of the internal-combustion engines. Then, as a method to clean exhaust gas while maintaining the performance of the internal-combustion engines, methods of reducing a content of incomplete combustion materials in the exhaust gas by improving the combustion efficiency of fuel have been proposed. The methods include an elevation of a combustion temperature of fuel, a removal of an impurity during a purification step of fuel, an increase of an octane number of fuels by an addition of additives or chemical reaction, and the like. However, the methods mentioned above generally make complicate productive facilities and manufacturing process, and are not practiced with ease and at low cost.